Here's a video of your game. Maybe I'm just getting retarded but I had a really hard time with it, I played sudoku before but the 4x4 grid and not being able to see the entire field really messed me up.
Viewing post in Sudokube (SteamGifts Demo) jam comments
Yeah, the 3D element really makes it harder. I'd suggest sticking with Beginner mode until you understand what's going on.
There's a huge gap in difficulty between Beginner and Easy because you can play Beginner mindlessly (only looking for solo numbers). When players get to Easy and realize they have to take their own notes, it tends to trip them up.